A gene which stops some ethnic groups getting rid of an alcohol by-product may be contributing to cancer cases.

It is well known that drinking heavily increases the chances of certain cancers, particularly those of upper digestive tract.

But some ethnic groups appear more prone to these cancers.

An international research project has found that a key genetic difference can make the saliva more carcinogenic.

Its work is reported in the journal "Alcoholism: clinical and experimental research".

Alcohol is toxic, and as soon as it enters the body, the liver starts trying to get rid of it by chemically breaking it down with natural body chemicals.

The theory is that it is not the alcohol which triggers the development of cancer, but a by-product of the way the body deals with the alcohol.

Gene missing

There are two stages to the breakdown - the first chemical converts alcohol to acetaldehyde, then a second chemical turns acetaldehyde to acetate, which can be dealt with more easily by other tissues outside the liver.

Some people lack the genetic code which lets the liver make the second chemical.

This means that such people have far more acetaldehyde in their bodies than people who have the necessary gene.

As many as 50% of Chinese and Japanese lack the gene. This causes obvious symptoms when they drink like facial flushing, dizziness and nausea, caused by the excess acetaldehyde.

But if these people drink more heavily, there is more of the toxic chemical in their saliva.

The study found that in the gene-deficient Asians, acetaldehyde levels in the saliva were two to three times higher than either Caucasians, or Asians who had the gene.

The scientists believe salivary acetaldehyde can cause cancer as it passes across the tissues lining the throat.

Mouth bacteria

Other things may increase the risk, the scientists suggest.

They believe bacteria living in the mouth may be able to break down alcohol into acetaldehyde - but no further.

So people with poor oral hygeine who drink alcohol may be placing themselves at higher risk.

Professor Ting-Kai Li, from the Indiana University School of Medicine said: "There's a high degree of suspicion or probability that acetaldehyde, which comes from alcohol, is carcinogenic, and this may be a mechanism in the higher rates of cancer among heavy drinkers who do not have this gene.

"It's not a one-to-one relationship, but it may increase the risk."

One of the study's authors, Dr Mikko Salaspuro, from Helsinki University Central Hospital, said: "Our findings open a new area, both for screening and preventative research, with respect to gastrointestinal tract cancer."

Dr Abdulla Badawy, from the Medical Council on Alcoholism, said that the proposals were "interesting and important", and merited further investigation.

The study comes as a survey from the Portman Group, a UK-based alcohol charity, suggested that up to 1m under-25s were "binge drinkers", who drank with the intention of getting drunk.

One in ten told Mori pollsters they did not feel happy at a social event without a drink in their hand.